Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.105
9. Slope Protection and Erosion Control. Although this is usually not the responsibility
of the grading contractor, upon completion of the fine grading,
slope protection and permanent erosion control devices are installed.
10. Trench Excavations. Utility trenches are excavated in the proposed road alignments
and building pads for the installation of the on-site utilities. The excavation
and compaction of utility trenches is often part of the grading process.
Once the utility lines are installed, scarifying and recompaction of the road
subgrade is performed and base material is placed and compacted.
11. Footing and Foundation Excavations. Although this is usually not part of the
grading operation, the footing and foundation elements are then excavated (see
Art. 6.9).
6.10.1 Compaction Fundamentals
An important part of the grading of the site often includes the compaction of fill.
Compaction is defined as the densification of a fill by mechanical means. This
physical process of getting the soil into a dense state can increase the shear strength,
decrease the compressibility, and decrease the permeability of the soil. There are
four basic factors that affect compaction:
1. Soil Type. Nonplastic (i.e., granular) soil, such as sands and gravels, can be
effectively compacted by using a vibrating or shaking type of compaction operation.
Plastic (i.e., cohesive) soil, such as silts and clays, is more difficult to
compact and requires a kneading or manipulation type of compaction operation.
If the soil contains oversize particles, such as coarse gravel and cobbles, these
particles tend to interfere with the compaction process and reduce the effectiveness
of compaction for the finer soil particles. Typical values of dry density for
different types of compacted soil are listed in Table 6.20.
2. Material Gradation. Those soils that have a well-graded grain size distribution
can generally be compacted into a denser state than a poorly graded soil that is
composed of soil particles of about the same size. For example, a well-graded
decomposed granite (DG) can have a maximum dry density of 2.2 Mg/m 3 (137
pcf), while a poorly graded sand can have a maximum dry density of only 1.6
Mg/m3 (100 pcf, Modified Proctor).
3. Water Content. The water content is an important parameter in the compaction
of soil. Water tends to lubricate the soil particles thus helping them slide into
dense arrangements. However, too much water and the soil becomes saturated
and often difficult to compact. There is an optimum water content at which the
soil can be compacted into its densest state for a given compaction energy.
Typical optimum moisture contents (Modified Proctor) for different soil types
are as follows:
(a) Clay of High Plasticity (CH): optimum moisture content � 18%
(b) Clay of Low Plasticity (CL): optimum moisture content � 12 to 18%
(c) Well-Graded Sand (SW): optimum moisture content � 10%
(d) Well-Graded Gravel (GW): optimum moisture content � 7%
Some soils may be relatively insensitive to compaction water content. For
example, open-graded gravels and clean coarse sands are so permeable that water
simply drains out of the soil or is forced out of the soil during the compaction
process. These types of soil can often be placed in a dry state and then vibrated
into dense particle arrangements.